Polymer electrolyte fuel cells that achieve low carbon dioxide emission and high electrical conversion efficiency are attracting a great deal of interest as clean energy systems of the next generation. The range of applications for such a fuel-cell, which can be provided at low cost by using a high-performance proton conductive polymer electrolyte membrane constituted of an inexpensive material, is diverse, including a power source for an electric car and a distribution type power source.
A solid-state polymer electrolyte fuel cell includes a fuel electrode constituted with one of the surfaces of a solid-state polymer electrolyte membrane with proton conductivity and an air electrode constituted with the other film surface. As a fuel gas containing hydrogen is supplied to the fuel electrode and an oxidizing gas containing oxygen such as air is supplied to the air electrode, a fuel electrode reaction whereby hydrogen molecules are separated into hydrogen ions (protons) and electrons occurs at the fuel electrode and an air electrode reaction whereby water is formed with oxygen, hydrogen ions and electrons occurs at the air electrode (see the following electrochemical reaction formulas) and as a result, an electromotive force is generated.Fuel electrode: H2->2H++2e−Air electrode: 2H++(½)O2+2e−->H2O
The solid-state polymer electrolyte membranes achieving proton conductivity in the related art include a perfluorocarbon sulfonic acid membrane (e.g., Nafion (product name) membrane manufactured by DuPont USA), a membrane constituted of a mixed material containing fluorocarbon sulfonic acid and polyvinylidene fluoride, a membrane obtained by grafting trifluoroethylene into a fluorocarbon matrix and a cation conductive membrane constituted with a cation exchange membrane of polystylene series having a sulfonic-group. When these solid-state polymer electrolyte membranes are wet, they work as proton conductive electrolytes. However, the properties of the solid-state polymer electrolyte membranes become altered at high temperature and the levels of their proton conductivity become lower. For this reason, water vapor is added to the gases supplied to the electrolytes and the operating temperature is controlled so as not to deviate from a low temperature range of 50 to 100° C.
As described above, the characteristics that a viable solid-state polymer electrolyte membrane is required to provide include; (1) superior proton conductivity, (2) easy management of the moisture contained in the electrolyte membrane and (3) a superior heat resisting property.
The characteristics requirements are addressed in patent reference literature 1, which discloses an electrolyte membrane manufactured by graft-polymerizing polyvinyl pyridine onto a base polymer that can be graft-polymerized and doping phosphoric acid onto the grafted base material. Patent reference literature 1 states that the resulting electrolyte membrane demonstrates superior proton conductivity at high temperatures equal to or greater than 100° C.
In addition, patent reference literature 2 discloses an electrolyte membrane achieving good proton conductivity under high temperature (150° C.) and low humidity conditions, by impregnating an acidic polymer (e.g., perfluorosulphonic acid) with a basic polymer (e.g., propylene glycol).
(Patent reference literature 1) Japanese Laid Open Patent Publication No. 2001-213987
(Patent reference literature 2) Japanese Laid Open Patent Publication No. 2001-236973
(Nonpatent reference literature 1) Takahito Itoh et al., “Ionic Conductivity of the Hyperbranched Polymer-Lithium Metal Salt Systems” J. of Power Sources, 81-82 (1999), p 824 to 829